In tablet formulation, it is important to accurately measure the hardness of a tablet containing a pharmaceutically active material, since the hardness of a tablet in part determines where the tablet will dissolve in the digestive tract. Tablet hardness is conventionally measured by determining the force required to fracture a tablet along its lengthwise axis. Numerous devices have been conventionally used to make such measurements. As general features, these prior art devices typically include a ram and anvil for crushing a tablet therebetween, and a strain gauge for measuring the amount of force required to fracture the tablet. U.S. Pat. Nos. 2,645,936, 2,041,869 and 2,975,630 disclose simple hand-operated hardness testers having these features.
More recent hardness testers include a frame so that the tester can be securely positioned on a work surface such as a table. U.S. Pat. No. 3,757,566 discloses a hardness tester wherein a tablet is placed in a groove defined between a pressure slide and a measuring slide. The tablet is crushed between these two slides, and the tablet powder must then be brushed away before another tablet can be tested. This prior art device has the disadvantage that tablet powder can work its way between the two slides, eventually fouling the mechanism.
U.S. Pat. No. 4,022,056 discloses a hardness tester wherein an anvil and hammer are disposed in a vertical relationship. This device has the disadvantages that the operator must again brush fragments and powder of the broken tablet off of the anvil, and delicately position the tablet on edge beneath the anvil.
U.S. Pat. No. 4,393,717 discloses an apparatus for measuring the thickness, diameter and hardness of medicinal tablets. According to this device, tablets to be tested are fed from a rotating carousel down an inclined trough onto a support. The tablet is then measured and crushed, and the broken tablet is removed by a scraper. This patent discloses a system which automatically tests a series of tablets, but the system disclosed by this patent is highly complicated, requiring a vibrator for causing a tablet to move down the trough, and a photosensing device for determining when a tablet has fallen from the end of the trough onto the support surface.
All of the foregoing prior art hardness testers share the problem that the fracturing zone of the tester must be cleaned before a new tablet can be tested. In the prior art, such cleaning is carried out by manual brushing of the apparatus, or by an automatic scraping device. None of the prior art hardness testers are self-cleaning in the sense that a separate operation for cleaning the fracturing zone is needed. Most conventional hardness testers are manually loaded, and incapable of carrying out a series of hardness measuring operations without human intervention.
It is an object of the present invention to provide a self-cleaning hardness tester which requires substantially no cleaning after a tablet has been fractured, by either a human operator or by an automatic device such as a scraper. It is a further object of the invention to provide an improved tablet dispenser for a hardness tester, which tablet dispenser is capable of reliably placing tablets one-by-one on a support surface for hardness testing, without need for complex structures such as a vibrating trough or photosensor. A further object of the invention is to provide a hardness tester which can be connected to a digital transducer indicator so that digital hardness readings can be obtained.